The present invention relates to communication systems, methods and devices used to communicate data between computers and in particular embodiments to telephone line coupled modem systems, methods and devices.
The need to communicate between distant computers has led to the use of telephone lines for data communication. The telephone lines are a natural choice for communications because of their ubiquitous nature and ability for dedicated instantaneous transmission between points. Modems are often used to communicate data between computers across a telephone line. A modem is a device that accepts digital data (for example, from a computer) and uses the data to modulate an analog signal for transmission across a telephone line. At the receiving end of the transmission another modem converts the analog information sent by the first computer and modem to digital data by demodulating the analog signal. The process of MODulating a signal on the sending end and DEModulating the signal on the receiving end is how the term xe2x80x9cMODEMxe2x80x9d was derived.
FIG. 1A illustrates a block diagram of a telephone modem coupled to a telephone line. In FIG. 1A the modem system 101 functionally comprises two blocks. The first block 103 is the actual telephone modem, which includes a telephone line interface circuit or Data Access Arrangement (DAA) 105. The second block 111 is the plain old telephone system (POTS) 111 which both accepts information from and provides information to the DAA 105 portion of the telephone modem 103. The telephone modem 103 commonly couples to the telephone system 111 via two telephone line terminals commonly denominated Tip or xe2x80x9cTxe2x80x9d 107 and Ring or xe2x80x9cRxe2x80x9d 109. The data access arrangement 105 provides the interface between the telephone modem 103 and the analog telephone system 111. The DAA 105 is typically an isolated Analog Front End (AFE), which the telephone modem uses to interface to the analog telephone system 111.
From the early days of telephone modems and telephone line equipment in general isolation is required between the telephone modem system 103 and the telephone system 111. The purpose of this requirement is to decouple any difference voltage potential between the telephone modem 103 and the telephone system 111. Furthermore, the isolation protects the user of the telephone modem from such things as lightning strikes within the telephone system 111, which could be destructive to the system and fatal to the user without adequate isolation. A transformer, such as 125 illustrated in FIG. 1B, were commonly used to address this isolation requirement. Typically, at least one driver, such as illustrated in FIG. 1B as 113, drives the transformer. Additionally, each driver circuit typically includes a resistor such as 121, which are used to set the impedance of the DAA seen by the phone line. The same transformer 125 may also be used for reception of signals. Signals are commonly coupled from the transformer 125 into a circuit known as a hybrid 119 and then further coupled into a receive amplifier 117. Generally, the function of the hybrid circuit 119 is to couple signals received from the telephone system to the receive amplifier 117 often after cancelling as much as possible of any transmit signal injected to the telephone line transmit buffer 113.
In the early days of modem development, the transformer 125 was used to carry DC xe2x80x9cloopxe2x80x9d current 129 from the telephone line as well as AC communication signals to and from the telephone line. Transformers with windings that carry DC current as well as AC signals are sometimes called xe2x80x9cwetxe2x80x9d transformers. The DC loop current 129 conducted by a wet transformer functions to inform the telephone system 111 that the modem is ready to communicate AC signals to and from a central office (CO) communications are impending. The process of causing a DC current in the telephone line is commonly referred to as going off-hook or seizing the telephone line. The magnitude of the DC current 129 used to inform the telephone system 111 that lines 109 and 107 are being seized is generally between 20 to 100 milliamps, depending on the distance of the modem system to the (CO).
Generally wet transformers used in modems had a limiting resistor 127, to set the DC resistance of the modem seen from the telephone line within specified limits. A typical current limiting resistor (e.g. 127) has a value of, for example, about 150 ohms and is typically placed in series with a primary winding of a transformer 125, which also commonly has a resistance of about 150 ohms. The addition of the transformer winding resistance and resistor resistance results in an additive DC resistance of approximately 300 ohms. The telephone line system 111 is, thus, presented with this 300 ohms resistance when a user goes off-hook.
An example arrangement for a wet transformer to provide off-hook current is shown in FIG. 2. The arrangement includes a relay 201, in series with the current limiting resistor 127. The closing of relay contact 201 couples the serial combination of the primary transformer 125 and resistor 127 to the telephone system. An advantage of a wet transformer system is that its primary winding is not polarized. Therefore, while FIG. 2 shows one example in which the resistor 127 side of the transformer 125 is coupled to the tip-line, coupling the resistor 127 to the ring side of the transformer would work equally well. Many early low-speed modems were configured with wet transformers.
However, wet transformers tend to exhibit nonlinear operation when DC current flows through the primary winding which can be problematic for higher speed modems. Because a transformer is a mechanical device, it is subject to such variations as magnetization, temperature variations and varying permeability. In addition, as the amount of current passing through its primary winding changes, so does the permeability of the transformer""s core. Modem systems generally function by detecting phase differences in incoming signals. As the speed of modem transmission increases above 2400 baud, modem systems became less tolerant of the distortion introduced by wet transformers, and wet transformers became less practical and more expensive to build than xe2x80x9cdryxe2x80x9d transformers for a specified linearity characteristic. FIG. 3A illustrates a dry transformer arrangement.
In FIG. 3, a direct current (DC) blocking capacitor 301 prevents DC from passing through the telephone line side primary winding of transformer 305. Because no DC passes through the primary of transformer 305, the linearity of the transformer 305 can be substantially improved over the wet transformer system, for the same physical size and cost. The dry transformer system does not inherently provide a path for the off-hook DC current however, and, so, another method is needed to provide the DC current for signaling the telephone system of an off-hook condition.
To draw off-hook current, a system referred to as an electronic inductor (EI) 303 was included with the dry transformer arrangement. An electronic inductor 303 has the ability to conduct off-hook DC current but appear as high AC impedance. It is beneficial for the electronic inductor 303 to appear as a high AC impedance so that the electronic inductor 303 does not contribute AC loading to either the transformer 305, or to the telephone system 311. The AC impedance value of the electronic inductor 303 is important because, in modern high-speed modems, an AC bandwidth from 10 hertz to 3.4 kilohertz is commonly desired and any impedance in parallel with the telephone line may affect the bandwidth of the signal and ultimately the performance of the system. Therefore, the electronic inductor must exhibit high impedance at frequencies from 10 hertz to 3.4 kilohertz and such as a coil, a passive inductor is not practical to use because its impedance is 2xcfx80 times the frequency of a signal, times the inductance. In order to use a passive inductor to provide high impedance to a 10-hertz signal, the inductance would be unacceptably large, and the physical size of the inductor impractical.
FIG. 4 illustrates an electronic inductor circuit 303. The DC current conducted by the electronic inductor is conducted primarily through the transistor 407 and resistor 405. The voltage at the junction 411 of resistors 401 and 403 determines the level of current through the transistor 407. The voltage at junction 411 is applied to the base of transistor 407, thereby holding the emitter of transistor 407 at a voltage equal to the voltage at 411 minus the voltage drop across the base-emitter junction of the transistor 407. Because the emitter-based voltage of transistor 407 is relatively constant, resistor 405 will determine the current through transistor 407 and consequently the current drawn by the electronic inductor 303 based on the bias voltage set at node 411. In order to keep the AC impedance of the electronic inductor high, a large capacitor 409 is provided at the base of transistor 407. To block AC signals, the series combination of capacitor 409 with the Thevenin equivalent of resistors 401 and 403 with capacitor 409 is the predominant factor determining the frequency response. Common exemplary values for resistor 407 and capacitor 409 are 50 Kxcexa9 and 10 uf, respectively. The serial RC combination of 401 and 409 provides high equivalent AC impedance for very low frequencies, such as 10 hertz.
Electronic inductor 303, however, still exhibits problems relating to the polarities on the lines 307 and 309. Transistor 407 is operational as an electronic inductor in FIG. 4 only if line 307 is positive (Ring) and line 309 is negative (Tip). In common telephone wiring, there is no guarantee which line is positive. Accordingly, a common approach is to use a diode bridge 501 to guarantee correct polarity, as shown in FIG. 5. Typical telephones and other telephone equipment in general also commonly use diode bridges to ensure correct polarity. By using a diode bridge 501, the ring signal, which is by definition positive, is directed into the collector of transistor 407 of the electronic inductor 303, or the positive terminal of EI 303.
Although some modern applications do not use transformers, most DAA circuits commonly include polarized circuit components, for instance transistors. Because the DAA circuits are polarized, diode bridges are commonly used to insure correct polarity. Diode bridges, however add to the overall circuit cost, and can also contribute to circuit noise.
Accordingly, to overcome limitations in the prior art described above, and to provide other advantages that will become apparent upon reading the present specification, preferred embodiments of the present invention relate to telephone line coupled devices, such as modem DAAs, which dispense with the need for a diode bridge.
A preferred embodiment of the present system involves a system for providing correct polarity signals without the use of a diode bridge.
In particular embodiments, the present invention provides a method and apparatus for switching incorrect polarity signals presented to a DAA.
A popular method for providing a modem is through the installation of a modem card into a personal computer. Modem cards inserted into a personal computer typically have two telephone jacks on one card. When installing a modem card, the user commonly connects one of the telephone jacks to the telephone outlet in the wall (or wall jack). The other telephone jack often provides a connection for a telephone, which may have previously been coupled to the wall jack and was disconnected to allow the modem card to be coupled to the wall jack. Such dual connectors on a personal computer modem card are commonly labeled xe2x80x9cphonexe2x80x9d and xe2x80x9cwall,xe2x80x9d xe2x80x9cphonexe2x80x9d and xe2x80x9clinexe2x80x9d or some other similar nomenclature. Although labeled separately, the telephone jacks on the modem card are often internally coupled together interchangeably such that there is no electrical difference between jacks. Embodiments of the present invention, which may be utilized in personal computer modems, provide for two telephone jacks wired in opposite polarity or a single jack whose polarity may be switched. Embodiments of the present invention provide that the tip signal on one jack will be the ring signal on the other jack and visa versa. In embodiments having 2 oppositely wired jacks, by plugging the telephone line into the correct jack, a correct polarity may be instituted in a DAA circuit or any polarized circuitry, without the use of a diode bridge. In embodiments having a jack with a switchable polarity if it is incorrect the correct polarity may be assured without the use of a diode bridge. Embodiments of the present invention provide, in addition to oppositely wired input jacks or switchable polarity, a method for insuring that the jack polarity is correct.